Hydrostatic transaxle

ABSTRACT

A hydrostatic transaxle axle assembly for a vehicle such as a grass mowing lawn or garden tractor comprising a housing for an internally disposed hydrostatic transmission having a variable-displacement hydraulic pump fluidly connected to a fixed-displacement hydraulic motor, the hydraulic motor being connected by differential gearing to axle shafts supported in the housing. The housing is formed by three housing members, two of which are preferably separable on a parting plane coincident with the longitudinal axes of the axle shafts. One of the housing members being provided with an opening to allow a portion of one of the other housing members to extend through, and where the extending housing member contains within its interior a number of internal fluid passages for fluidly connecting the hydraulic pump to the hydraulic motor. This construction allows for improved cooling of the power transmitting fluid flowing between the pump and motor and simplifies manufacture of the transaxle such that the bulk or all the machining operations can be carried out in only one of the three housing elements, this housing member being the smallest of the three which can be completed using a comparatively smaller sized CNC machining-center than would otherwise be the case. The two remaining housing elements may be used in an as cast or supplied condition.

RELATED APPLICATIONS

[0001] This application is a division of co-pending application Ser. No.09/411,835, filed on Oct. 4, 1999, which is a division of U.S.application Ser. No. 09/112,363, filed on Jul. 9, 1998 which issued intoU.S. Pat. No. 5,979,270, which claims the benefit under 35 U.S.C.§119(e) of Provisional Application No. 60/051,990, filed on Jul. 9,1997, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to hydrostatic transaxles, and moreparticularly, to a compact, fully integrated hydrostatic transaxleincorporating a hydrostatic transmission of the type employing an axialpiston swash-plate pump and a hydraulic motor. Hydrostatic-transmissionsin association with transaxle driving apparatus have proven to be veryuseful to-date and are used in numerous applications such as smallvehicles like self-propelled grass-mowing lawn tractors.

BACKGROUND OF THE INVENTION

[0003] Hydrostatically powered driven equipment such as lawn tractorshave become extremely popular and many utilise the axial pistonswash-plate configuration for both the pump and motor elements of thehydrostatic transmission. Such tractors generally have an internalcombustion engine having a vertical crankshaft which is connected to thetransaxle by means of a conventional belt and pulley arrangement. Astandard hydrostatic transmission for such a transaxle includes ahydraulic pump, which is driven by an input shaft from the engine outputby means of the belt and pulley arrangement, and a hydraulic motor, bothpump and motor are mounted on a center section located inside thetransaxle housing. Rotation of the pump by an input shaft creates axialmotion of the pump pistons during periods when the pistons are operatingagainst an inclined thrust or swash-plate. The fluid flow thus createdby the reciprocating axial motion of the pistons is channelled viaporting and passages in the center section to the hydraulic motor, withthe effect that the incoming fluid causes the pistons of the motor toreciprocate and create a turning moment that causes rotation of thehydraulic motor. The hydraulic motor in turn has an output shaft whichdrives the vehicle's axles through speed-reducing gears and a mechanicaldifferential. Examples of such hydrostatic transaxles are shown in thefollowing patents: U.S. Pat. No. 5,090,949; No. 5,473,964 and No.5,501,640.

[0004] All three references use an axial piston swash-plate pump andmotor respectively engaged to a center section which is located within atwo-shell housing structure. The main purpose of the center section isto provide a fluid link between the pump and the motor and allowing thetransmission of hydraulic power. Patents '964 and '640 in the names ofOkada and Hauser respectively, teach the use of an input shaft drivenpump where the swash-plate lies adjacent to the upper housing. Thiscontrasts with the disclosure in patent '949 which teaches the use ofbevel gears for connecting the input-shaft to the pump and wherein thisexample of prior art, the swash-plate of the pump lies directly acrossboth the upper and lower housings of the transaxle.

[0005] The center section shown in all three above references requirenumerous machining operations to prepare the initial raw casting to beready for use. For instance: drilling some or all of the internal flowpassages and arranging retaining means so that plugs/valves and suchlike can be subsequently fitted to close off the flow circuit; makinggood two of the faces which provide the fluid coupling means for thepump and motor, and when required, for the subsequent attachment of thevalve-plates; preparation of mounting surfaces for attaching the centersection to the housing structure. Furthermore, the upper transaxlehousing aluminium diecasting itself requires a number of machiningoperations before it can be used such as the provision for the shaftbearing and seal as well as hole or holes and seals for the controllever and various associated linkages.

[0006] As a general rule, the more machining operations required in theupper transaxle housing casting as well as the more complex operationsrequired in the center section casting, the greater the cost ofmanufacture of the complete hydrostatic transaxle. Therefore thereduction in the number of such machining operations and by groupingthem into one rather than two components would save expense.

[0007] Although only shown in the '949 patent, almost all hydrostatictransaxles make use of a cooling fan mounted to the input drive shaft inan attempt to help prevent the internal components and fluid fromoverheating. However, the prior art teaches a center section whichalthough attached in some manner to the interior of the housing, it isstill essentially a separate entity from the transaxle housing. As aresult, effective cooling of the fluid passing through the passages inthe center section that connect the pump and motor together is hinderedas the fluid surrounding the center section acts as a insulator to slowdown the rate of heat transfer from the power transmission fluid in saidpassages to the surrounding housing radiator.

[0008] The amount of heat able to be radiated away from the transaxlehousing exterior to the surrounding environment is of course greatlyenhanced over that region on the boundary of the transaxle housing thatlies directly in the path of the air flow from the cooling fan. However,it is apparent that although the fluid inside the housing nearest thatregion where the fan is operating is being cooled, fluid elsewhere maystill remain at very high temperature. Perhaps more importantly, as thefluid circulating between the pump and motor in the fluid passages inthe center section becomes extremely hot during operation, especiallywhen the unit is heavily loaded and used in a high ambient temperatureenvironment, the resulting drop of operating efficiency due todecreasing fluid viscosity and a corresponding increase in fluid leakagelosses can be a concern with the prior art.

[0009] This problem exists because the attendant power losses associatedwith such close coupled pump and motor combinations produce a lot ofunwanted heat due to the rapid fluid compression/decompression cyclesand general friction between the sliding surfaces; Such losses causesthe fluid circulating between the pump and motor through the centersection to become extremely hot, and because the prior art teaches atransaxle housing structure whereby the internal fluid reservoircompletely or almost completely surrounds and insulates the centersection, these prior solutions are not conducive to the promotion ofmost effective cooling for the circulating fluid in the centre sectionflowing in a closed-loop circuit between the pump and motor. Thislimitation occurs because the bulk of the heat accumulating in thecenter section can only be transferred by conduction to the surroundinghydraulic fluid and then through the fluid itself to reach the boundarywalls of the housing surrounding the fluid chamber from where it can beradiated away to the surroundings. The remove of unwanted heat from thecenter section consequently takes time.

[0010] Therefore in these prior devices where the center section iseffectively insulated by the surrounding hydraulic fluid, the delay inthe transfer of unwanted heat out of the transaxle may on occasionresult in the fluid of the hydrostatic transmission becoming overheatedwith the risk that the operational life of the fluid is shortened orthat the lubricating properties of the fluid deteriorates to the extentthat threatens the useful operational life of the hydrostatic transaxle.

SUMMARY OF THE INVENTION

[0011] An object of the invention is to eliminate the need to use aconventional centre section in the hydrostatic transaxle product.

[0012] A further object of the invention is to provide a hydrostatictransaxle in which most or all the machining can be carried out in onehousing member compared to the prior art where such operations need tobe carried out in both the housing as well as the center section.

[0013] A still further object of the invention is the grouping of thefluid coupling surfaces for the pump and motor as well as the heatdissipating means on the exterior surfaces on a single housingcomponent, including fluid passages arranged in its interior withbearings, seals, control shafts supported in machined pockets.

[0014] A further object of the invention is to improve the cooling ofthe operating fluid circulating between the pump and motor. With thisinvention, fan cooling of the housing very effective in lowering thetemperature of the hydrostatic transmission fluid as it circulates inthe closed-loop circuit between the pump and motor as much of the heatis conducted directly into the material of the housing in the areadirectly under the path of the air from the cooling fan.

[0015] What is needed in thee art is a compact hydrostatic transaxle forvertical input shaft installations where the amount of machining neededto be carried out is consigned to one rather than two or three maincomponent members, preferable in a manner that would allow both theupper and the lower transaxle housing elements or shells for thehydrostatic transaxle to be used in an as cast or supplied state. Whatis further needed in the art is improved cooling for such devicesallowing an extension in the operation duty cycle.

[0016] In one form thereof, the hydrostatic transaxle of the inventioncomprises an axle assembly with a housing having an internal chamber foran internally disposed hydrostatic transmission and forming an integralpart of said hydrostatic transmission, said hydrostatic transmissioncomprising a variable-displacement hydraulic pump and afixed-displacement hydraulic motor; axle shafts rotatably supported insaid housing and differential gearing means supported by said housingdrivingly connected between said hydraulic motor and said axle shafts;said housing comprising first, second and third housing members andwhere said first and second housing members are joined together on afirst parting plane and where said second and third housing members arejoined on a second parting plane and wherein said first housing memberincludes internal fluid passages arranged to fluidly connect saidhydraulic pump to said hydraulic motor.

[0017] The above mentioned and other novel features and objects of theinvention, and the manner of attaining them, may be performed in variousways and will now be described by way of examples with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is an external view from one side of the hydrostatictransaxle according to the invention.

[0019]FIG. 2 is a plan view of the hydrostatic transaxle of FIG. 1 alongthe section line I-I.

[0020]FIG. 3 is a sectioned view on line II-II of FIG. 2 showing thehydrostatic transmission.

[0021]FIG. 4 is a further sectioned view on line III-III of FIG. 2.

[0022]FIG. 5 is a view of an alternative housing construction for thehydrostatic transaxle of FIG. 1.

[0023]FIG. 6 is a part sectional view along line IV-IV of FIG. 5.

[0024]FIG. 7 depicts external control linkages for use with thealternative housing construction of FIG. 5.

[0025]FIG. 8 is a part sectional view along line V-V of FIG. 7.

[0026]FIG. 9 is a sectioned view taken at line VI-VI of FIG. 8.

[0027]FIG. 10 depict various forms of fluid valves for use in thealternative housing construction.

DETAILED DESCRIPTION OF THE FIRST EMBODIMENT OF THE INVENTION

[0028] The first embodiment shown in FIGS. 1 to 4, the outer housingstructure of the hydrostatic transaxle depicted by arrow 1 is partiallydefined by an upper transaxle housing element 2 and a lower transaxlehousing element 3 which are joined together at a junction surfacearranged preferably such that the parting-plane S is coincident with theoutput axle shafts 7, 8. Parting-plane can be called the secondparting-plane in this invention. A liquid gasket seal is applied to thejunction surface at the parting plane 5 prior to the two transaxlehousings element 2, 3 being secured together by a plurality of bolts orscrews 10. As shown in FIGS. 3 & 4, upper transaxle housing element 2 isarranged to have opening 12 through which housing member 13 passes andwhich is then secured in-place by a plurality of fastening screws 15. Aseal ring or liquid gasket seal is applied to the junction surface 16between housings members 2, 13 next to opening 12. Junction surface 16can be called the first parting-plane in this invention. All threehousing members 2, 3, 13 when attached together form the surroundingboundary for an internal chamber 17. Within internal chamber 17,internal elements of the hydrostatic transaxle are located therein suchas the hydraulic pump, hydraulic motor, speed reducing gears andmechanical differential. Although internal chamber 17 may be dividedinto separate chambers for purposes of segregating the hydrostatictransmission from the mechanical gearing (and when required a mechanicaldifferential), the embodiment here illustrates the use of a commonchamber 17 for all such components.

[0029] Housing member 13 may preferably be cast as one-piece in eitheraluminium allow or iron/steel. If the piece is cast as a pressurediecasting, many features can be detailed with sufficient accuracy suchthat many secondary machining operations can be thus avoided.

[0030] As a portion of housing member 13 containing within it the fluidpassages connecting the pump and motor is exposed to the outerenvironment, porosity in the casting could led to fluid leakage.However, the possibility of such fluid seepage through the material ofthe housing member in the present invention can be easily overcome in atleast one way, for instance, through the impregnating of that portion ofthe housing member that would protrude through the opening in the upperhousing member of the transaxle with a resin that prevents leakageoccurring.

[0031] Housing member 13 contains within its structure fluid passages20, 21, 22, 23 that connect together the respective fluid couplingsurfaces otherwise known as valve-faces, between the pump and motor.Fluid passages 20, 21, 22, 23 may be cored in the housing casting 13 ormachined. Plugs 25 are used to blank off the ends of passages 20, 21.Although in practice it has been known to cast valve faces in aluminiumalloy to provide a running surface for a cylinder-barrel, most commonpractise these days is to mount separate valve-plates on whichrespective cylinder-barrels can run against. Most often, such separatevalve-plates are surface hardened so that the sealing surfaces mayoperate for many thousands of hours without suffering from undue wearthat would result in a loss in volumetric efficiency of the hydrostaticunit. In this respect, the faces shown as 28, 29 on the housing member13 may be cast with countersunk register shown as 30, 31 respectively,into which respective valve plates 33, 34 are located. However, theinvention would permit the valve faces to be detailed and formeddirectly on the surface of the subsidiary housing 13 if desired.

[0032] Check-valves are included in both respective pairs of passages20, 22 and 21, 23 to allow the admittance of makeup fluid in order thatthe hydrostatic transmissions can recover any fluid loss duringoperation because of leakage. In the present invention, the check-valvesshown as 37 have been placed near the lowest position in the downwardlyextending portion shown as 40 of the housing member 13 such that balls42 of the check-valves 37 can fall onto their respective seats 44 by theinfluence of gravity when not activated by suction pressure. Althoughnot shown, the linkage connecting the swash-plate of the pump to theexternally protruding control-lever of the hydrostatic transaxle may bearranged so that when the pump is at or near neutral, the linkage canact in a manner to off seat the balls, thereby proving the hydrostatictransmission with a more positive neutral point.

[0033] The respective cylinder-barrels 60, 61 of thehydrostatic-transmission pump and motor are mounted perpendicular to oneanother such that the rotating axis of the pump cylinder-barrel 60 isvertical and arranged parallel and coaxial with respect to theinput-drive shaft 62 whereas the rotating axis of the motorcylinder-barrel 61 is parallel with respect to the rotating axis of theaxle-shafts 7, 8. The input drive shaft 62 is supported by a bearing 63in the housing element 13 and a seal 64 is used to prevent fluid in theinternal chamber 17 from escaping. Shaft 62 extends downwards and isprovided with a spline 65 which connects with the pump cylinder-barrel60. Shaft 62 extends further and passes through the center of theswash-plate 70 to be further supported by means of a bearing 72 in lowerhousing member 3.

[0034] Bearing 72 is supported in blind hole 71 which ideally may besized by the die-caster thereby eliminating any need to machine lowerhousing member by the transaxle builder although in practice.Alternatively, hole 71 could be cast as a through hole and in this case,an extra seal would then be needed. Bearing 72 may be of theself-aligning type in which case a plain journal bearing may be added tothe design positioned, this bearing being positioned just adjacent tovalve-plate 33 to act between bearings 63, 72 for providing additionalsupport for drive-shaft 62.

[0035] To overcome any misalignment between the bearing 63 in housingmember 13 and bearing 72 in the lower housing member 3, it is proposedthat during assembly of the hydrostatic transaxle, housing member 13 isonly loosely attached by screws 15 until all the components have beenassembled in place. Then once input shaft 62 is located into bearing 72,this essentially controls the true position of the housing 13 relativeto housing members 2, 3 as the bearing 63 supported input shaft 62 isthereby in correct alignment with bearings 72. At that time, screws 15can be tightened so that housing elements 2, 13 are thereby lockedtogether.

[0036] The cylinder-barrel 60 of the pump is provided with a pluralityof axial cylinder-bores 75, each bore 75 containing a respective piston76 and where each piston 76 is being axially urged outwards by a spring(not shown) located behind the piston 76 in the bore 75. The outer endof the piston 76 is generally domed-shaped to be operatively connectedto an adjacent operating surface 80 of swash-plate 70 by the biasproduced by springs. The action of the springs behind each of thepistons 76 produces a counter reaction which loads the cylinder-barrel60 against the operating surface 81 of the adjacent valve-plate 33. Eachcylinder-bore 75 has a port 77 so arranged to communicate in sequencewith a pair of arcuate-shaped ports (although not visible they areidentical to those arcuate-shaped ports 98, 99 shown for the motor)provided on the valve-plate 33, and where such arcuate-shaped ports arein fluid connection with fluid passages 20, 21 in housing member 13.

[0037] Swash-plate 70 is so arranged for the pump that its inclinationangle can be varied in both directions from its neutral orzero-inclination point. The swash-plate 70 is seated on apart-cylindrical bearing 88 provided in the interior of the lowerhousing member 3, and connected by linkages (not shown) to acontrol-shaft 90 which protrudes from housing member 2 as shown inFIG. 1. Rotary movement of the control-shaft 90 causes the swash-plate70 to incline in angle in respect of the stroking axis of the pistons76, and thereby the stroke of the pistons 76 is changed. The amount ofpiston 76 stroke determines the amount of fluid is placed in thecylinder-bore 75 per each single rotation of the cylinder-barrel 60, andhence the swept volume of the pump can be changed by altering the amountof piston stroke so that the amount of fluid delivered to the hydraulicmotor is precisely controlled.

[0038] The cylinder-barrel 61 of the motor is almost in all respectsidentical to that of the pump, and carries a series of pistons 93 whichare operatively connected to the operational surface 94 of thrust plate95. Each piston 93 is housed in its respective cylinder-bore 96 providedwithin cylinder-barrel 61, and arranged so that each cylinder-bore 96can communicate in sequence with respective arcuate-shaped ports shownas 98, 99 provided in the valve-plate 34 by means of its respective port97. Fluid passages 23, 23 in the housing member 13 are arranged to be influid connection with respective arcuate shaped ports 98, 99 in thevalve-plate 34 of the motor by means of respective linking ducts 91, 92.Although not shown, a spring is positioned within each of thecylinder-bores 96 to engage with its respective piston.

[0039] In the case of the motor, the thrust-plate 95 is depicted in itsmost often used position which is permanently inclined with respect ofthe axis of pistons 93. The thrust plate 95 is supported on a insert 100that is held to the housing member 2, 3 either by fastening mens such asscrews or preferably by being pinched tight between adjacent walls asshown 101, 102 in housing member 2, 3. Because the inclination angle. ofthe thrust-plate 95 always remains at an angle during the operation ofthe device, the piston 93 stroke in the motor remain constant. As thefluid received from the pistons 76 of the pump can be changed by theaction of using control-shaft 90, the rotation of the motor can be ineither direction. The cylinder-barrel 61 is mechanically engaged byspline 105 to shaft 106 and thus rotation of the cylinder-barrel 61causes rotation of shaft 106. As fluid enters the motor from the arcuateshaped ports 98, 99 in the valve-plate 34, the fluid entering thecylinders of the pistons 93 causes the pistons 93 to move axiallyoutwards and because the reaction on the thrust-plate 95 to the pistonmovement is not co-axial with the longitudinal axis of the pistons 93,an angular driving moment is created on the cylinder-barrel 61 which iscaused to revolve. Therefore rotation of the cylinder-barrel 61 andshaft 106 is transmitted through the speed reducing gears to thedifferential and axle output shafts 7, 8 of the hydrostatic transaxle 1which in the case of a vehicle application such as lawn tractor, areattached the drive wheels of the vehicle.

[0040] Shaft 106 is supported by bearings 110, 111, bearing 110 beinglocated in the downwards extending portion 40 of subsidiary housingmember 13, whereas bearing 111 is located in a pocket 112 arrangedbetween housing member 2, 3. The shaft 106 may protrude from thetransaxle so that a conventional disc parking brake can be attached. Aseal 114 is also provided to surround the shaft 106 in order to preventsfluid seeping out of the internal chamber 17.

[0041] A gear 120 fixed to shaft 106 is in mesh with gear 123 which isfixed to intermediary-shaft 125. Intermediary shaft 125 is supported bybearings 127, 128 in similar manner to that described for the shaft 106.Gear 130 fixed to intermediary-shaft 125 is in mesh with the ring-gear131 of the differential-assembly.

[0042] The ring-gear 131 of the differential-assembly has bevel gearsshown as 132, 133, 134, 135 so that power can be transmitted from thedifferential-assembly to the axle-shafts 7, 8 of the transaxle 1 asknown to those skilled in the art. The inclusion of a differentialassembly is important as it allows normal differentiation between theleft and right drive wheels of the vehicle and helps prevent lawn damageespecially when tight turns are undertaken However, there areapplications where no such differentialled action is required, and inthese instances, a single axle shaft may be used instead of the two asshown in this embodiment. In the case of a single axle shaft, this shaftcan be arranged to extend outwardly on one or both sides form thehousing.

[0043] By means of appropriate selection or adjustment of theinclination of the swash-plate 70 of the pump by means of thecontrol-shaft 90 and the intermediary linkage, the hydrostatictransmission ratio is altered. Rotation of the input-shaft 62 causescylinder-barrel 60 to rotate and results in reciprocation of pistons 76.Fluid is then delivered from passages 20, 22 or 21, 23 (depending onwhich direction of low occurs from the pump) in the housing member 13and enters the cylinder-barrel 61 of the motor and causes the pistons 93to reciprocate in their respective bores by way of their angle of attackagainst the inclined thrust-plate 95. A side force is created by thepistons 93 on the wall of each bore in those bores subjected topressurised fluid, causing rotation of the motor cylinder-barrel 61about its longitudinal or central axis for rotation. The cylinder-barrel61 in turn rotates shaft 106 and mechanical power is transmitted throughgears 10, 123, 130 to the ring gear 131 of the differential. Bevelgearing of the differential then determines the respective speeds of theaxle output shafts 7, 8 that drive the wheels of the vehicle.

DETAILED DESCRIPTION OF THE SECOND EMBODIMENT OF THE INVENTION

[0044] As the second embodiment differs in only one main respect fromthe first embodiment, description is only necessary to show the mainpoints of differences. As the internal components are identical to thosedescribed for the first embodiment, for convenience, most that are herenumbered will carry the same reference numeral as for the firstembodiment.

[0045] Essentially as shown in FIGS. 5 to 9, the housing member 200containing within the pairs of fluid passages 201, 203 and 202, 204 thatare used in linking the pump to the motor, is attached to an exteriorjunction surface (first parting-plane) shown as 210 provided on theupper surface of housing member 212. Housing element 212 is providedwith an opening 215 which acts as the register for housing member 200and where fastening screws 217 are used to secure housing members 200,212 together. Housing member 212 combine with the lower transaxlehousing member 213 to form an internal chamber 220.

[0046] Housing member 200 is provided with a face surface 221 for thevalve-plate of the pump, and on the downwards extending portion 205 isprovided with face surface 222 for the valve-plate 34 of the motor.Check-valves show as 225 are included for each passages 203, 204respectively, and plugs 226, 227 as shown in FIG. 9, are used to closethe ends of passages 201, 202. Plugs 226, 227 do not need to be threadedin passages 201, 202 as they are prevented from being expelled becauseof adjacent wall formed by the opening 215 in housing member 212.

[0047] This embodiment also contrasts with the first embodiment in thatsubstantially more cooling fins shown as 233 can be included on the topsurface 234 of housing member 200, thereby providing more efficient andeffective cooling of the fluid passing through passage 201, 202, 203,204 between the pump and motor.

[0048] Although the present invention will still allow some of the heatto be transferred out from the transaxle by conduction through thehydraulic fluid medium to the outer housing in a similar manner as usedin the prior art devices, the total cooling effect is enhanced becausethe most important area to be cooled, namely the fluid path between pumpand motor, is directly adjacent that portion in the housing on which thecooling fan is most effective. Consequently, the hydrostatic transaxlecan be operated for longer periods at rated loads with less risk ofoverheating the power transmission fluid.

[0049] A control-shaft 251 is journalled at 252 in the housing member200. Having the control shaft located in the housing member containingthe internal fluid passages can also be performed in the firstembodiment of the invention if so desired.

[0050] Rotary vent valve 260 shown in FIGS. 7 to 10 may be used toprovided a “wider band neutral” effect for the hydrostatic transmission,and as well as if desired, a “freewheel” or fluid dumping effect.Essentially, the valve 260 is supported in bore 261 in housing member200 and is provided with a fluid short-circuit shown as small passage263 which, when the swash-plate of the pump is near its zero inclinationangle, passage can connect with two passages shown as 265, 266 inhousing member 200 which are in communication with fluid passages 201,203 and 202, 204 respectively. As shown, any fluid released by the pumpwhen the swash-plate has a small inclination angle can divert throughpassage 263 rather than flow to the hydraulic motor where it would actin causing the motor assembly to rotate. FIG. 7 shows how valve 260 canbe linked to control-shaft 251 by means of linkages 270, 271, 272 sothat the movement in position of passage 263 occurs as control-shaft 251is moved by the operator of the vehicle. By varying in the span or armlength of the linkages, it is possible to obtain varying characteristicsfrom the “wider band neutral” to suit each particular application. Valve260 is shown as the left-hand illustration in FIG. 10 and shows hiddenpassage 269 which is perpendicular to passage 263. Disconnection oflinkage 270 with valve 260 would allow the valve 260 to be rotatedthrough ninety degrees to provide a large flow short-circuit betweenpassages 265, 266 as hidden passage 269 becomes a short-circuit for thefluid. Valve 280 shows a modification whereby grooves 281, 282 areprovided for “O” ring type seals that surround passage 263. Valve 290illustrates a further modification where the valve can be axially liftedso that passage 263 is no-longer in connection with passages 265, 266 inhousing member 200, so that in the lifted position, slot 291 is open topassages 265, 266 to short-circuit fluid into internal chamber 220. Thusthe degree of flow restriction caused by fluid having to pass throughthe restricted passage 263 is overrided, and the vehicle can be easypushed without having to first start the engine. A further advantage ofhaving the vent valve in the housing element is that adjustments to theoperating condition of the hydrostatic transaxle can be made withoutdisassembly of the entire unit.

[0051] Although not shown or described in either of the embodiments ofthe invention, a further modification falling within the scope of thepresent invention would be to rearrange lower housing member shown as 3in the area about the swash-plate 70 and bearing 72 so that a fourthhousing member could be used. The fourth housing member would beprovided with a hole for bearing 72 and a part-cylindricalsupport-surface for the swash-plate 70. The fourth housing member couldbe attached and secured to the inside interior wall of housing element 3or alternatively, be attached to the outside of housing element 3 on amounting face and arranged that part of it extends through an openingprovided the housing member 3. In this manner, the housing membercontaining the fluid passages can further be attached to the fourthhousing element by means of stays or studs which are arranged to passthrough the top exterior to the bottom exterior which would stiffen thecomplete housing structure of the hydrostatic transaxle.

[0052] Perhaps for certain applications, there may be advantage insubstituting the fixed-displacement axial piston swash-plate hydraulicmotor with that of another type. For instance, a fixed-displacementexternal geared hydraulic motor could be used instead and where thegears of the motor in this instance would be journalled in boresprovided in the same housing member containing the fluid passageslinking the pump to the motor.

[0053] A charge and/or power take-off auxiliary pump, preferably of thegerotor internal gear type may also be disposed in the same housingmember that contains the fluid passages linking the pump to the motor ofthe hydrostatic transmission. The gerotor pump being driven by the inputdrive shaft and having fluid passages and valves arranged in the samehousing member to suit the needs of the application. A furtheradvantages in this arrangement would allow the pressure setting of thegerotor pump to be adjusted easily as the pressure relief-valve wouldhave an external adjustment; the fan cooling effect of the presentinvention would help keep the fluid delivered by the gerotor pump to bekept as cool as possible; all the external connections can be arrangedto be near the top of the transaxle thereby minimising the changes ofbeing damaged.

[0054] In accordance with the patent statutes, we have described theprinciples of construction and operation of our invention, and while wehave endeavoured to set forth the best embodiments thereof, we desire tohave it understood that obvious changes may be made within the scope ofthe following claims without departing from the spirit of our invention.

We claim:
 1. An integrated hydrostatic transaxle comprising a housing; ahydrostatic transmission disposed in said housing, said hydrostatictransmission comprising an axial piston pump having a rotary pumpcylinder barrel, and an axial piston motor having a rotary motorcylinder barrel; an input drive shaft having a vertical axis of rotationrotatably supported in said housing and having a first portionprotruding outside of said housing and a second portion protrudinginside of said housing to engage said rotary pump cylinder barrel by wayof spline connection for rotation at equal speed; a motor output shafthaving a horizontal axis of rotation rotatably supported in said housingand said engaging said rotary motor cylinder barrel by way of splineconnection for rotation at equal speed; a mechanical differentialdisposed in said housing; an internal bulkhead wall formed in saidhousing and formed substantially vertically to be disposed generallyparallel to said vertical axis of rotation of said input drive shaft,said internal bulkhead wall delimiting an inner fluid reservoirsurrounding said hydrostatic transmission from an inner fluid reservoirsurrounding said mechanical differential such that said hydrostatictransmission is disposed one side of said internal bulkhead wall whereassaid mechanical differential is disposed on the opposite side; axleshafts rotatably supported in said housing and extending outwardly fromsaid housing, said mechanical differential drivingly connected betweensaid motor output shaft and said axle shafts; and a fluid couplingsurface disposed in a first horizontal plane inside said housing andengaging said pump cylinder barrel, said fluid coupling surface beingspaced from underlying surfaces of said housing, said motor output shafthaving a horizontal axis of rotation disposed in a second horizontalplane, said first horizontal plane being parallel to and not coincidentwith said second horizontal plane and wherein said first horizontalplane is situated nearer said first portion of said input drive shaftthan said second horizontal plane.
 2. The integrated hydrostatictransaxle according to claim 1 wherein said housing includes a lowertransaxle housing element and an upper transaxle housing element joinedtogether along a horizontal parting plane, said upper transaxle housingelement including transverse channels opening on said parting plane toreceive said axle shafts.
 3. The integrated hydrostatic transaxleaccording to claim 2 wherein the longitudinal axis of rotation of saidaxle shafts is disposed parallel to said parting plane.
 4. Theintegrated hydrostatic transaxle according to claim 3, and furthercomprising reduction gearing disposed in said housing and residing inthe same said inner fluid reservoir surrounding said mechanicaldifferential housing, said reduction gearing drivingly connecting saidmotor output shaft with said mechanical differential and including agear fixedly mounted on said motor output shaft, said motor output shaftarranged to pass through said internal bulkhead wall to connect saidgear to said rotary motor cylinder barrel for rotation at equal speed.5. The integrated hydrostatic transaxle according to claim 4 whereinsaid transverse channels communicate with said inner fluid reservoirsurrounding said mechanical differential.
 6. The integrated hydrostatictransaxle according to claim 5 wherein said lower transaxle housingelement and said upper transaxle housing element once combined togethersubstantially surround said mechanical differential, and wherein each ofsaid axle shafts is supported in said housing by a pair of axle supportlocations, one of said axle support locations being disposed adjacentsaid mechanical differential and the other of axle support locationsbeing positioned closer to where the respective axle shaft protrudes outfrom said housing.
 7. The integrated hydrostatic transaxle according toclaim 6, and further comprising a further fluid coupling surfacedisposed inside said housing, said further fluid coupling surfaceengaging with said motor cylinder barrel; a pair of fluid passagesinside said housing and hydraulically connecting said fluid couplingsurface and said further fluid coupling surface together; a check valvedisposed in each said pair of fluid passages and where at least one saidcheck valve is disposed on that side of said fluid coupling surfacewhich is furtherest away from said first portion of said input driveshaft.
 8. The integrated hydrostatic transaxle according to claim 7wherein said second horizontal-plane is coincident with said partingplane.
 9. The integrated hydrostatic transaxle according to claim 8wherein said parting plane is uniplanar.
 10. The integrated hydrostatictransaxle according to claim 9, further comprising transverse channelsin said lower housing element and opening on said parting plane forreceiving said axle shafts, and wherein said longitudinal axis of saidaxle shafts is coincident with said parting plane.
 11. The integratedhydrostatic transaxle according to claim 1 wherein said internalbulkhead wall fluidly segregates said inner fluid reservoir surroundingsaid hydrostatic transmission from said inner fluid reservoirsurrounding said mechanical differential and said reduction gearing. 12.The integrated hydrostatic transaxle according to claim 1, wherein saidinternal bulkhead wall permits fluid to flow between said inner fluidreservoir surrounding said hydrostatic transmission and said inner fluidreservoir surrounding said mechanical differential and said reductionbearing.